TÉNÉRIFE, Spain — It rains little on this island. There are no natural rivers, and the air is full of the dry heat of the nearby Sahara.

But in a ravine on the island's northern tip, tree limbs drip with water and a tropical forest flourishes, sustained almost entirely by condensation from the low-lying clouds that are regularly pinned up against the mountainside.

The area, called Cruz del Carmen, is only one example of the unusual evolutionary habitats on Spain's Canary Islands that fascinated Charles Darwin about 200 years ago, and that today reveal a new species or subspecies to scientists an average of once every six days.

But the unique plant and animal life here is being steadily overtaken by an invasion of foreign species, which have been entering the islands in increasing numbers since border checkpoints within the European Union were abolished under the Schengen agreement a decade ago, according to government officials and scientists here.

"There is very potent legislation controlling what enters into Europe," José Luis Martín Esquivel, an adviser to the regional government on environmental issues, said in an interview. "But there are very weak controls on what crosses the borders within Europe."

Government officials on these islands, which are about 115 kilometers, or 70 miles, off Morocco's southwestern coast, say that an invasive species enters their borders at the rate of once every 17 days, and that one turns into a plague capable of altering native habitats and seriously threatening native species an average of once every six months.

"With the Schengen agreement, more and more plagues are coming in," Domingo Berriel, the director of the regional government's environmental protection agency, said in an interview. "We need the power to control our borders more closely, or it could be true disaster."

Berriel said his main concern was that the invasive species may seriously endanger the islands' huge population of endemic species, which exist naturally nowhere else on earth.

Over all, about 4,000 endemic species have been recorded on the Canary Islands, compared with about 100 for all of Britain, according to the regional government. But the true number is expected to be considerably larger, because dozens of new species are found every year.

The natural wonders of these islands have drawn researchers for centuries. Darwin dreamed of a visit in the 1830s. His "Canary scheme," as he called it, was far enough along that he began studying Spanish, immersed himself in scientific writings on the islands, and prepared to reserve a ticket on a ship that would soon be heading here.

But months before he was to come, he was appointed to a position on the Beagle and went to the Galápagos Islands instead.

"People here like to say that if he had come, he would have developed his theory of evolution here," said Agustín Aguiar Clavijo, a zoologist with the regional government.

The richness of the islands' biodiversity is most evident in Cruz del Carmen, which has the most endemic species per square kilometer in Europe, according to a study by the regional government.

Although Cruz del Carmen is home to a tropical forest, the temperatures here are often chilly and sunlight is often blurred by dense fog, leading the plant and animal life down a unique evolutionary course.

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A random handful of soil here can easily contain four or five endemic species of insects, Aguiar said. "This is our great jewel," he said. "This is a marvel of evolutionary marvels."

Elizabeth Ojeda Land, a botanist with the regional government, said the area had so far escaped significant damage from invasive species. "It is a miracle that a zone like this has been preserved, with all the tourists that come to the islands and the population density we have," she said.

But other areas have been less lucky. The regional government estimates that invasive species have already cut the percentage of endemic plant species on the islands in half in the past 50 years, from one of every two plants to one of every four.

Among the most notorious invasive species here is the Barbary ground squirrel, which has decimated plants on the island of Fuerteventura, and the highly aggressive Argentine ant, whose fast-growing colonies have expelled native ants and other insects from their natural habitats.

The most recent threat comes from a beetle called the picudo rojo, which has been ravaging many of the islands' signature Canary palm trees for the past several months. The beetle appears to have spread from nonnative palm trees that were recently brought into the islands.

But perhaps the biggest problem for endemic plants like the Canary palm is not invasive insects or animals but the threat of hybridization from close contact with nonnative species that are brought to the islands.

The dual threat from hybridization and invasive pests has meant that plants fill most of the spots on the Canary Islands' most endangered species list.

The regional government says about 20 endemic species are currently endangered. Several have fewer than 100 specimens left in the wild, and some have fewer than two dozen.

Ojeda, the botanist, said she was working to preserve a plant species that had only three surviving specimens.

Many other species with relatively large numbers of specimens are also at risk because all of them live within a single confined area, often less than a square mile. If the habitat within that area is significantly altered, the species are likely to become extinct, the government says.

Berriel, the director of the regional government's environmental protection agency, said the best way to manage the threat to local habitats was to give the regional government the authority to set up customs checkpoints at its borders.

"If we don't act soon, the consequences for our endemic species could be grave," he said.